Specifying Validation Order
Attribute validation occurs when focus moves from the attribute, whereas entity validation occurs when the record is committed. With compositions, changing a child entity causes the parent entity to be validated as well, with child entities being validated before parent entities.
You can specify the validation order of rules within a certain group (entity or an attribute) by using the up and down arrows to rearrange the validation rules on the Validators page of the editor. For example, although you cannot control the order in which attributes are validated (they are validated in the order in which they appear in the entity definition), you can order validations for a given attribute.
If you want to ensure that a Key Exists or entity-level Method validator is performed at the end of the validation process, you can set it to perform at transaction level, rather than at entity level. It is then fired after all entity-level validation is performed. You set the level of validation on the Validation execution tab of a validation rule editor.